A filamentous clone of Polysiphonia urceolata was regenerated from segments cut from the fronds of gametophytes. Unlike wild thalli with short virgate branchlets, the clone was filamentous with few branches. Many transparent trichoblasts arose from pericentral cells during the induction culture, but these were seldom observed during normal growth. The trichoblasts were uniseriate, often colorless, and formed lobed rhizoids rapidly when they came into contact with solid substrates. In addition to morphological characteristics, the photosynthetic properties and growth conditions of the clone differed from those of the mother plant. Cross-gradient light and temperature culture experiments revealed that the most favorable conditions for culture of the filamentous clone were 22°C and 95-120 μE/(m2-s) light intensity. The photosynthetic light saturation value for filaments was approx. 100 μE/(m2-s), which is far lower than that of wild thalli. These results could be used to develop techniques for mass cultures of P. urceolata in photobioreactors for production of seed stock or bioactive products. Unlike wild thalli was reproduced from segments cut from the fronds of gametophytes. Unlike wild thalli with short virgate branchlets, from the fronds of gametophytes. Many transparent trichoblasts arose from pericentral cells during the induction culture, but these were seldom observed During normal growth. The trichoblasts were uniseriate, often colorless, and formed lobed rhizones rapidly when they came into contact with solid substrates. -gradient light and temperature culture experiments revealed that the most favorable conditions for culture were at 22 ° C and 95-120 μE / (m2-s) light intensity. The photosynthetic light saturation value for filaments was approx. 100 μE / (m2-s), which is far lower than that of wild thalli. These results could be used to develop techni ques for mass cultures of P. urceolata in photobioreactors for production of seed stock or bioactive products.